1. Field
The present disclosure relates to a system for rotating a rotor in a gas turbine engine.
2. Description of the Related Art
FIG. 1A illustrates an aircraft engine comprising a fan 100, a low pressure (LP) compressor 102, a fan case 104, an engine casing 106, a High Pressure (HP) compressor 108, a HP turbine 110, a LP turbine 112, and a LP shaft 114 connecting the LP compressor 102 and the LP turbine 112. After engine shutdown on the ground, residual hot air 116 in the engine core rises 118 and is trapped by the engine casing 106. As the hot air rises 118, the upper portion 120 of the HP compressor's (engine's rotor) 108 rotor shaft 122 becomes hotter than the lower portion 124 of the rotor shaft 122 and causes uneven cooling and thermal deformation of the engine rotor shaft 122 (i.e., rotor bowing, where the upper portion 120 of the rotor shaft 122 becomes longer than the lower portion 124). Upon engine restart (e.g., prior to fuel ignition in the combustor 126), even tiny fractions of rotor shaft bowing can cause the HP compressor (engine's rotor) 108 to rub against the engine's casing 106. The rub causes vibrations (manifested as disconcerting noise in the aircraft cabin) or even damage to the aircraft (e.g., engine damage, damage to the engine case lining, damage to the air pre-cooler used by the environmental control system, or damage to other accessories). Also illustrated are the rotor shaft's 122 longitudinal axis 128, transmission 130 connecting a gearbox 132 to the rotor shaft 122, and air 134 inputted into the engine during operation. FIG. 1B illustrates that on an aircraft 136, the engine 138 is housed in a nacelle 140. The nacelle 140 may also trap rising 118 hot air 116 that causes a temperature gradient T.
One method to mitigate these problems is to build the engine with wider cold build clearances (“opened up” clearances), so that the compressor rotor shaft 122 can bow without causing blades to rub on the engine casing 106. However, more advanced engine designs prefer less “gap” between the engine casing and the compressor rotors (tighter “cold build clearances”) to reduce air leakage and improve thrust specific fuel consumption (TSFC). Thus, the overriding need to reduce fuel consumption renders wider cold build clearances less desirable. Indeed, as ever tighter cold build clearances are implemented, the problems caused by engine rub will become more severe.
Conceivably, an engine architecture could add rotor stiffening or bearing arrangements to reduce the amount of rotor shaft bow that is physically possible. However, these architecture changes would add weight and manufacturing cost to the engine.
Other methods of mitigating rotor shaft bow comprise rotating the shaft (1) so that the shaft cools uniformly, returns to thermal equilibrium, and straightens, and/or (2) so that centrifugal forces straighten the bow. The shaft rotation is achieved (1) by motoring the engine at relatively low revolutions per minute (RPM) after starting the engine (but before running the engine at high RPM) and/or (2) using an Engine Turning Motor (ETM) to turn the rotor shaft when the engine is off.
However, conventional methods for providing power to the ETM or the engine so as to straighten the bow can be problematic. Some smaller aircraft, such as the Boeing 737 airplane, fly into remote airports where facility power is not available to power the ETM or engine. Furthermore, auxiliary power unit (APU) power on the aircraft is not always available to power the engine or ETM because some airports limit APU use at gates due to emissions and noise concerns and aircraft are not powered when they are towed between gates. In addition, airplanes may operate with a nonfunctional APU or the powering of the ETM or engine may cause undesirable APU wear (extended motoring prolongs the APU's exposure to main engine start (MES) mode, reducing APU life). Finally, the use of lithium-ion and nickel-cadmium batteries for powering the ETM is problematic due to high failure rates and flammability concerns associated with the engine environment (extreme heat, extreme cold, and high vibration).
Moreover, rotating the shaft shortly before departure causes departure delays, especially if reduced engine clearances require turning the rotor at low speeds. These delays not only inconvenience the passengers but also increase costs associated with increased waiting times and parking fees.
What is needed then, is a more efficient method for mitigating rotor shaft bowing that simplifies ground logistics. The present disclosure satisfies this need.